The principle governing the operation of the read sensor in a magnetic disk storage device is the change of resistivity of certain materials in the presence of a magnetic field (MR or magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
Referring now to FIG. 1, the key elements of what is termed a top spin valve are, starting at the lowest level, seed layer 1, free magnetic layer 2, non-magnetic spacer layer 3, magnetically pinned layer 4, pinning layer 5, and capping layer 6. Inverted structures in which the free layer is at the top are also possible (and are termed bottom spin valves). To isolate the device from extraneous magnetic fields it is sandwiched between two magnetic shields 11 and 17. Also seen in FIG. 1 are the conductive leads 15 that attach to the device.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. As first shown by Barkhausen in 1919, magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to single-domain films for the MR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps and during normal head operation. This is most commonly accomplished by giving the structure a permanent longitudinal bias provided by two opposing permanent magnets. In FIG. 1 the longitudinal bias is provided by a laminate of ferromagnetic layer 13 (typically nickel-iron) and antiferromagnetic layer 14. An alternative way to provide the longitudinal bias is to use a layer of a magnetically hard material. This is shown as layer 21 in FIG. 2.
As track density requirements for disk drives have grown more aggressive, GMR devices have been pushed to narrower track widths to match the track pitch of the drive and to thinner free layers to maintain high output in spite of the reduction in track width. Narrower track widths degrade stability as the device aspect ratio starts suffering. Thinner free layers have traditionally degraded stability and increased the asymmetry distribution across the slider population. Hard-bias of the type described above, that is typically used to overcome stability concerns associated with the junction, also results in amplitude loss due to the field originating from the hard bias structure. Side reading, which is attributable to any deviation of the head microtrack profile from a square, also gets worse with narrower track widths
With increased track density, the dead zone (which is defined as the area between the physical and magnetic read widths) in a conventional contiguous junction has been decreasing as the physical dimension has continued to shrink. At approximately 0.3 microns the dead zone becomes negative implying that the magnetic read width (MRW) is larger than the physical track width (PRW) dimension. Hence, for track width dimension of 0.3 microns and less, it is possible to retain more than half the readback amplitude with more than half the read head placed outside the written track.
This effect is due in part to the fact that the track width has been scaling down faster than other dimensions such as shield-to-shield (S-S) spacing (18a and 18b) and fly height. Shown in FIGS. 1 and 2 are, respectively, typical exchange bias and hard bias contiguous junctions in use with GMR devices. Modeling has shown that the side reading is reduced by using lower fly heights and thinner S-S. This implies that part of the side reading is due to the stripe edges and how they pick up flux from adjacent tracks. The topography for a typical head further increases the S-S spacing at track edges since the shield to shield (18b) needs to be increased to accommodate the lead and stabilization thickness.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,198,608, Hong et al. show a contiguous junction GMR device. U.S. Pat. No. 5,818,685 (Thayamballi et al.) construct a biasing magnet by using multiple layers of ferromagnetic material separated by non-magnetic layers. U.S. Pat. No. 6,185,078 B1 (Lin et al.) and U.S. Pat. No. 5,739,987 (Yuan et al.) are related MR processes.